Mass spectrometry can be used for qualitative and quantitative identification of compounds in a wide variety of samples, including metabolomics, proteomics, pesticide analysis, natural substance identification, pharmaceuticals and comparable fields. Liquid Chromatography-Mass Spectrometry (LC/MS) is particularly used in such analyses.
In this area, the recognition of isotopic patterns is often considered useful. The control of a mass spectrometer based on detected isotopic fingerprints (patterns in the mass spectrum) is also known. Examples of this are shown in: Drexler, D. M. et al., “Automated Identification of Isotopically Labeled Pesticides and Metabolites by Intelligent ‘Real Time’ Liquid Chromatography Tandem Mass Spectrometry using a Bench-top Ion Trap Mass Spectrometer”, Rapid Commun. Mass Spectrom., 1998, 12, 1501-1507; Chernushevich, I. V. et al., “An introduction to quadrupole-time-of-flight mass spectrometry”, J. Mass Spectrom., 2001, 36, 849-865; Lock C. et al., “ICAT Labeled Protein Analysis via Automated Liquid Chromatography/Orthogonal MALDI QqTof”, Proceedings of the 49th ASMS Conference on Mass Spectrometry and Allied Topics, May 27-31, 2001; and U.S. Pat. No. 7,189,964.
These techniques often rely on strong isotopic signals from components like Chlorine or Bromine, where the contribution to the overall isotopic pattern from heavy isotopes is significant (>30% for chlorine and >80% for bromine). Without high resolution, it becomes difficult to separate fine structure in the spectrum. Fine structure here can be defined as the ability to separate the members of the nominal parts of the isotopic pattern (A1, A2, A3. etc.) into their constituent parts, which are contributed by the specific atoms that make up the observed species. The small mass differences in the isotopes of carbon, hydrogen, nitrogen, oxygen, sulphur, chlorine, bromine and other atoms and their abundances (either natural or artificial) are the source of this fine isotopic structure.
High resolution mass spectrometry is commonly used for quantitation of pollutants. This may be performed using double-focusing sector mass spectrometry, for example. The high resolution can differentiate between peaks from different sources having the same nominal mass. An example of this is shown in WO2010/025834, having common ownership with this invention.
More recent developments have begun to use high resolution mass spectrometry to overcome the difficulties in recognising isotopic patterns. EP 2 128 791 discusses the comparison of isotopic patterns with simulated isotope patterns, in order to guide an analysis of elemental composition. Stoll, N. et al., “Isotope Pattern Evaluation for the Reduction of Elemental Compositions Assigned to High-Resolution Mass Spectral Data from Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry”, J. Am. Soc. Mass Spectrom., 2006, 17, p. 1692-1699 discusses the use of isotopic fine structure for pruning of elemental composition candidate lists (see especially FIGS. 4 and p. 1696, col. 2). Also, quantitative isotopic fine structure analysis is also known in isotope ratio analysis, although dominantly with the goal of avoiding interferences. This is shown in EP 1 770 779, especially for geological applications.
For detection of metabolites, a so-called “mass defect analysis” or “Kendrick mass analysis” is frequently used. Various aspects of this method are discussed in U.S. Pat. Nos. 8,237,106, 8,063,357, 7,634,364 and 7,381,568. Essentially, by identifying ions with a certain class of exact mass defects, it is expected to catch metabolic derivatives of particular known substances. These methods directly use a single exact mass for identification of members of a substance class.
All of these approaches (but especially the isotopic fingerprinting approach and the approach for filtering by “mass defect”) are focussed on identification of the complete elemental composition of a compound, molecule or fragment. Whilst the mass defect approach can identify the presence of a single functional group, this is still limited to analysis of individual molecules. An analysis that considers the entire mass spectrum is significantly more difficult.